Centrifugal apparatus and method for processing explosives

ABSTRACT

Primary explosives, e.g. lead azide, dispersed in a liquid medium, e.g.  enol, are filtered, washed and dried in a novel centrifuge, wherein the rotatable extractor basket has an open bottom, which is closed by peripheral attachment to the large open end of an invertible funnel. The small open end of the funnel is attached to an axial shaft, which can be raised and lowered, whereby said small open end can be held upright to retain the contents in said basket and lowered to invert the funnel to discharge the dried free-flowing explosive particles by gravity through said small open end.

GOVERNMENTAL INTEREST

The invention described herein was made under a contract with the U.S.Government

This is a division of application Ser. No. 507,821, filed Sept. 20,1974.

BACKGROUND OF THE INVENTION

The present invention is directed to improvements in the current"backline" operations, wherein primary explosives, e.g. lead azide, leadstyphnate and tetracene, are transferred from shipping containers,processed, i.e. filtered, washed and dried, and transferred to the bowlsused in loading of detonators and similar devices.

The backline operations are essentially the same for all primaryexplosives and these procedures have changed only slightly since theseexplosives were first used in military items. The current process isrelatively time-consuming, expensive, and because of the nature of theprocess is hazardous to personnel working on the backline.

At present, the backline operation is a batch process performed manuallyin a series of separated small buildings using laboratory sizedequipment. Lead azide and lead styphanate are shipped in drums with upto 150 pounds of explosive contained in a large rubber (or plastic) bagpacked in sawdust wet with a water-alcohol solution in an outer burlapbag. The large rubber (or plastic) bag contains from 2 to 14 bags ofexplosives (the number and weight of explosives in these inner bagsdepends on the vendor) commonly containing from 10 to 25 pounds ofexplosives.

Tetracene is shipped in a 30-gallon metal container with two 12.5-poundbags of tetracene inside one large bag. Packing material is the same asfor lead azide and lead styphnate, sawdust, and a water-alcoholsolution.

In present operations, the outer bag is opened, sawdust and waterremoved, the rubber (or plastic) bag opened, and an inner bag istransferred to a container containing enough liquid to cover the bag.When ready to process, the bag (kept wet with alcohol) is kneaded tobreak up lumps and the explosive is transferred from the bag. Allpacking material is returned to the shipping drum and a kill solutionadded. After an appropriate waiting period, the drum is moved to adisposal area and the contents discarded.

After removal from the bag, approximately 2 pounds of the explosive istransferred (in some cases directly from the bag) to a large Buchnerfilter, the water-alcohol removed and the resultant cake washed withethanol to remove the water, taking care to ensure that the wash liquidgoes through the explosive and that explosive is not allowed to dry outin the filter. The damp explosive is transferred to drying tubes aftermanually breaking up the filter cake.

After drying at 120° F., the explosive is transferred to the screeningbuilding where it is remotely transferred to a conical cloth screen(jelly bag). Screening is accomplished by up and down movement of theconical cloth screen and the screened explosive goes into anapproximately 2-pound container. The next operation in the case of leadazide to be used in detonators and similar items is rebowling in which 2pounds of lead azide is transferred to the smaller cups used on thedetonator loading line or machines. Lead azide is to be used in blends;lead styphnate and tetracene are not rebowled and stay in the 2-poundcontainers until weighed remotely.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved method andapparatus for filtering, washing and drying primary explosives, whichavoids the hazardous, costly and time-consuming operations associatedwith operations currently practiced in the art.

In accordance with the present invention, primary explosives, such aslead azide and the like, which are slurried in a liquid medium, e.g.aqueous ethanol, are filtered, washed and dried in a novel centrifuge,wherein the extractor basket is provided with an invertible funnel-likecontainer for collecting and discharging the dried explosive produced asfree-flowing particles. The extractor basket has an open bottom, whichis closed by peripheral attachment to the large open end of thefunnel-like container, which has a small open end attached to an axiallyaligned movable shaft. During operation of the centrifuge the shaftholds the small open end of the funnel upright, whereby the contents areretained in the basket and container. At the end of the drying cycle,the shaft moves downwardly to invert the funnel, thereby permitting thedried, free-flowing explosive particles produced to discharge by gravitythrough the small open end of the funnel. The centrifuge is providedwith conduits for introducing the slurry, wash liquor and drying gasinto the extractor basket.

By means of the present method and apparatus, primary explosives can befiltered, washed and dried rapidly and safely to produce and discharge afree-flowing product, which does not require screening and can bedirectly loaded into "scooping" bowls for final loading operations.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view in partial cutaway of an embodiment ofthe novel centrifuge apparatus of the present invention.

FIG. 2 is a perspective, cutaway view of a novel centrifuge apparatus ofthe type illustrated in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the figures, wherein like reference numerals refer to likeparts, a cylindrical stainless steel extractor basket 10 having porouswall 11 is positioned within a cylindrical residue pan or bowl 12 ofstainless steel having drain ports 14 and 16. The extractor basketcontains a lip 18 for preventing spillover of contents and is attachedto a wheel-like member comprising a hub 20, which is welded to a hollowtubular vertical drive shaft 22 and has four symmetrically spaced spokemembers 24 and 26 (only two are shown) attached to a ring 28. Aninvertible funnel-like inner container or boot 30 of a strong, flexiblesilicone rubber material is peripherally held at its large open end 32between the wheel ring 28 and a boot retaining ring 34 by means ofmachine screws 36, which also attach said rings to the rim 37 of thecylindrical basket 10 thereby closing the open bottom of basket 10. Thesmall end 38 of boot 30 is provided with discharge ports 40 and isattached to the lower end of hollow dump shaft 42, which possesses acoaxial passage 43. Dump shaft 42 is coaxially positioned in hollowdrive shaft 22 and provided with bushings 44. The upper end of shaft 42is held in a bell-shaped housing 45 attached by bolts 46 to a bearingretainer 47, which contains ball bearings 48 and is provided withprojections 49, which ride in guide channels 50 in frame 51. Dump shaft42 is supported by retainer ring 52 and rotates with drive shaft 22 towhich it is connected by said funnel and wheel members. The upper end ofthe housing 45 is fastened by a lock nut 53 to the threaded arm 54 of apneumatically operated piston 55 in cylinder 56 adapted to raise andlower the dump shaft 42. A tray 57 containing openings 58 is attached tobowl cover assembly 60 by means of four equally spaced screws 61 and 62and wing nuts 63 and 64 (only two of each are shown). A conduit 66 isprovided for introducing liquid explosive slurry and wash liquor at theinner wall of extractor basket 10. A conduit 68 is connected to coverbowl assembly 60 for introducing heated drying gas into the plenumthrough the openings 58 of tray 57, and is provided with a branchconduit 70 for directing part of such drying gas at the inner wall ofthe extractor basket. Drive shaft 22 is rotated by attached gear 72,which engages the drive gear 74 of motor 76. Shaft 22 is also providedwith self-aligning ball bearings 78 (not detailed) and is supported inposition by flange 80 which rests on shoulder block 82 mounted on shelf84 of support structure 85. The centrifuge bowl cover assembly 60 ismounted on shelf 86 of support structure 85 with four screws 88, 89(only two are shown), while motor 76 and frame 51 are mounted on shelf84 with screws 90 and 92, respectively (not all are shown). A vibrator98 is mounted on shelf 86 to transmit vibrations to the extractor basket10 for dislodging and discharging adhering dry explosive particles fromsaid basket and funnel-like boot 30. A conduit 94 is connected to thebell-shaped housing 45 for introducing a stream of air through thecoaxial passage 43 of dump shaft 42 and lateral ports 96 in the lowerpart thereof, which communicate with the clearance provided between theouter wall of dump shaft 42 and the inner walls of hollow wheel hub 20and drive shaft 22, whereby any particles of explosive entering saidclearance during processing are blown back into the basket by the airstream and thus prevented from forming dangerous accumulations and/orpenetration into friction areas between the inner shaft 42, outer shaft22, and hollow wheel hub 20.

In operation the extractor basket is rotated at high speed via driveshaft 22. During the filtration, washing and drying cycles thefunnel-like boot 30 is held in upright position within basket 10 by dumpshaft 42, as shown in FIG. 1. The slurry of solid explosive particles isfed through conduit 66 into the extractor basket 10, wherein the liquidis removed through the pores of wail 11 and the solid explosiveparticles are retained on said wall. A suitable wash liquid is thenintroduced through conduit 66 to wash the explosive and to displace theliquor adhering to the solid explosive, after which heated air, nitrogenor other suitable gas is introduced through conduits 68 and 70 to effectdrying of the solid explosive to a free-flowing powder. The size, shapeand position of the orifices of the liquid and gas conduits within thebasket can be adjusted to provide the maximum contact of the wash liquorand drying gas with the solid explosive. At the conclusion of the dryingcycle, the funnel 30 is inverted to the position shown by dotted linesin FIG. 1 by forcing dump shaft 42 downward by means of pneumaticallyoperated piston 55, whereby the free-flowing powdered explosive dropsthrough discharge ports 40 at the narrow end of funnel 30 to a loadingbowl 99 or other suitable receiver used in loading of primaryexplosives, detonators, etc.

The centrifugal extractor basket, which can have any suitable size andconfiguration, e.g. bowl or cylindrical shape, possesses an open bottomsuch that the dried explosive particles can be discharged therefrom whenthe aforesaid flexible funnel-like container attached to the open bottomthereof is inverted. The funnel-like container can be peripherallyattached in any suitable manner at its large open end to the open bottomof the extractor basket and at its small open end to the shaft adaptedto invert same so as to discharge the dry explosive particles therefromat the end of the final cycle. The funnel-like container can befabricated of a flexible material, e.g. natural or synthetic rubber,cloth, etc., preferably a "conductive" rubber or a material whosesurface has been treated so as to reduce buildup of static electricity,which could otherwise be hazardous in view of the possibility that thecontainer might rub against metal parts.

Primary explosives, particularly lead azide which is the primaryexplosive used in the largest quantities, are generally loaded in frontline operations by automatic machinery or at hand-loading stations intodetonators, etc., from a container containing 2 ounces or less of suchexplosive. The following example illustrates the use of the presentinvention in which lead azide is processed in separate portions ofapproximately 50 grams each (about 2 ounces), whereby safety ofoperation is improved without loss of efficiency or quality of explosiveproduct produced, as compared with current backline operations.

The centrifuge of the foregoing type employed in the example possessed acylindrical extractor basket which had an inside diameter of 4.5 inchesand pores of 10 mu diameter, and was rotated at 1000 rpm. Dextrinatedlead azide, as received, was mechanically mixed with 95% ethanol toproduce a uniform slurry containing approximately 5% by weight of thelead azide. About one liter of the slurry containing 50 grams of thelead azide was transferred into the centrifuge during one minute through1/2 inch I.D. Tygon Tubing by means of a peristaltic pump. When thetransfer was complete, the basket was spun for 30 seconds. A wash of 500ml. of anhydrous methanol was then pumped into the basket during 30seconds to displace adhering ethanol and water, after which the basketwas spun for 30 seconds. A wash of 500 ml. of trichlorotrifluoroethane,marketed under the trademark Freon 113 by E.I. du Pont de Nemours & Co.,was then pumped into the basket during 30 seconds, after which thebasket was spun for 2 minutes while introducing a current of hot air(about 180° F.) to dry the lead azide. The rotation of the basket wasthen stopped, and the contents thereof were discharged in about 15seconds by inverting the funnel-like container and vibrating thecentrifuge, as described above. The lead azide was recovered essentiallyquantitatively as a free-flowing powder free from agglomerates. Theamount of lead azide lost in the waste wash liquors was negligible (Itvaried from 0.04 to 0.08 grams per liter of waste wash liquor over anumber of repetitive runs in the foregoing manner, based measurement ofthe observed gas evolution after addition of an excess of ceric ammoniumnitrate).

The characteristics of the lead azide crystals thus produced weresimilar to those of lead azide obtained by normal backline methods. Whenthe product was incorporated in a 50% lead azide - 50% aluminum blendused as the output charge for a delay primer, no loading problems wereencountered and the output of the item was the same as that of the itemcontaining normally processed dextrinated lead azide.

The total time required for processing a slurry of 2 ounces ofdextrinated lead azide to a dry, free-flowing product as described abovewas approximately 5 minutes. A longer processing time was required whenthe Freon wash was omitted and/or the drying gas employed was heated toa lower temperature.

When one liter containing 20 grams of tetracene slurried in ethanol wasprocessed in the foregoing manner except that the final spin wasincreased to about 2.5 minutes, the tetracene was recovered essentiallyquantitatively as a free-flowing product.

In another experiment one liter contining 25 grams of lead styphnateslurried in ethanol was processed in the foregoing manner except that95% ethanol was employed as wash liquid in place of anhydrous methanoland the final spin period was increased to about 4 minutes. In two runsmade in this modified manner, 25.0 grams of free-flowing lead styphnateproduct were dumped at the end of the final cycle in the first run and24.0 grams in the second.

It is thus evident that by using several centrifuges of the presentinvention and processing small (e.g. 2 ounce) batches of slurriedexplosive in the manner described above, it is possible to process a bagcontaining up to 25 pounds of primary explosive per hour to consistentlyproduce a dry, free-flowing explosive equal to that obtained by presentbackline operations while substantially reducing operating whilesubstantially and hazards. The individual centrifuges and storage tankswith associated equipment, e.g. stirrers, pumps, etc., can be separatedbehind protective shielding. After transfer of the explosive fromshipping bags to the batch tanks, the subsequent operations required toproduce about 2 ounces of dry, free-flowing primary explosive forloading into explosive items can be performed remotely andautomatically.

The foregoing disclosure and drawings are merely illustrative of theprinciples of this invention and are not to be interpreted in a limitingsense. I wish it to be understood that I do not desire to be limited toexact details of construction shown and described for obviousmodifications will occur to a person skilled in the art.

What is claimed is:
 1. A method for processing a liquid slurry ofprimary explosive to produce a dry, free-flowing explosive product freefrom agglomerates, which comprises:a. introducing the liquid slurry ofprimary explosive into a centrifugal extractor basket wherein theexplosive solids are retained and the liquid medium is removed, saidextractor basket having an open bottom which is closed by beingperipherally attached to the large open end of a flexible invertiblefunnel whose small open end is held upright to retain the contents ofsaid basket; b. introducing wash liquid to displace adhering liquidslurry medium from the explosive solids; c. introducing gaseous dryingmedium to remove adhering liquid from said explosive solids to produce adry, free-flowing product free from agglomerates; and d. inverting theflexible funnel to discharge said product through the small open end ofsaid funnel,said basket being rotated during the processing to effectremoval of liquid.
 2. The method of claim 1, wherein the primaryexplosive slurry consists essentially of a slurry of a primary explosiveselected from the group consisting of lead azide, tetracene and leadstyphnate in aqueous ethanol and the wash liquid consists essentially ofan alcohol selected from the group consisting of anhydrous methanol and95% ethanol.
 3. The method of claim 2, wherein the explosive slurryconsists essentially of a slurry of lead azide in aqueous ethanol andthe wash liquid consisting essentially anhydrous methanol.